Incentive for betting on ancient city (4)
I believe the answer is:
spur
'incentive' is the definition.
(I've seen this before)
'betting on ancient city' is the wordplay.
'betting' becomes 'sp'.
'on' is a charade indicator (letters next to each other).
'ancient city' becomes 'ur' (ancient Sumerian city state).
'sp'+'ur'='SPUR'
'for' acts as a link.
(Other definitions for spur that I've seen before include "Cowboy boot attachment" , "branch road" , "Encourage" , "Incentive; branch line" , "slip road" .)